Conventionally, there is proposed an electrical power system for grid connection, which includes a power generator and an electrical storage device, installed in a residential power system (for example, Japanese Patent Application Publication No. 2002-369406 (hereinafter referred to as “Document 1”)). Document 1 discloses an electrical storage means (an electrical storage device) for storing generating energy from a solar battery (a power generator), and also describes that energy from a power supply system or generating energy from the solar battery is stored in the electrical storage means.
The generating energy from the solar battery is not only stored in the electrical storage means but also supplied to electric loads. Document 1 describes an energy selling (reverse power flow) condition for supplying the power supply system with the generating energy from the solar battery when energy supplied to the electric loads is smaller than the generating energy from the solar battery. The electrical storage means discharges the energy stored therein so as to prevent electric power of the power supply system from exceeding prescribed power. That is, in the technology described in Document 1, energy is allowed to flow back to the power supply system when the generating energy from the power generator exceeds demand energy for the electric loads and thereby excess energy is produced.
A power generator that generates electricity from natural energy, such as a solar battery is thought as an equipment with less environmental load, because no carbon dioxide is emitted when it generates electricity. In the existing circumstances, the introduction of this sort of power generators are prompted by setting a unit price of the generating energy flowing back to the power supply system, namely a selling price of electricity higher than a purchasing price of electricity from the power supply system, thereby giving returns by selling electricity.
In the technology described in Document 1, when the generating energy from the solar battery exceeds the demand energy for the electric loads, it is regarded that excess energy is produced, and the energy is allowed to flow back to the power supply system. The excess energy is however hard to be produced when a variation pattern of the generating energy is similar to that of the demand energy and the difference between the generating energy and the demand energy is small, because whether or not excess energy is produced is defined based on time variation between the generating energy and the demand energy. That is, some consumers can hardly obtain returns by reverse power flow owing to little excess energy.
Thus, the consumers with little excess energy may have a problem that it takes long years to recover investment costs for installing power generators. Increasing excess energy in order to collect such investment costs for a short period of time requires a change in daily rhythm so as to reduce demand energy in the daytime, so that adverse effects may occur in daily life.